Poly-p-phenylenebenzobisthiazole, poly-p-phenylenebenzobisoxazole and poly-2,5-benzoxazole are intractable polymers by which is meant they are non-melting but soluble. Mixtures of these polymers have been made with thermoplastic polymers to provide melt-processability. Mixtures of these polymers with other intractable polymers have been made for various purposes. Procedures for making these mixtures have, in general, required isolation of the intractable polymer from its polymerization solvent, polyphosphoric acid, and its combination with a solution of the other polymer is methanesulfonic acid. The ternary composition, namely, the two polymers and single solvent were then extruded to form fiber.